This invention relates generally to telecommunications networks, and more particularly, to a method and system for improved fast forward power control in a CDMA network.
As known in the art, forward link processing for the power control subchannel gain is handled in the following manner. At the base station controller (BSC), for each call, the BSC monitors whether or not a soft handoff state has changed. If there was no soft handoff state change, the BSC then continues to monitor. If the BSC detects that the soft handoff state has changed, then the BSC determines whether or not a change is required for the forward power control subchannel gain (FPC_SUBCHAN_GAIN). If no change is required for FPC_SUBCHAN_GAIN, then the BSC returns to the step of monitoring whether or not a soft handoff state has changed.
If a change is required for FPC_SUBCHAN_GAIN, then the BSC sends a message to the relevant base transceiver stations (BTSs) in soft handoff with the mobile station to increase/decrease the power of the punctured bits by a predetermined amount relative to the power of full rate (i.e., 9600 bps or 14400 bps) frames. The BSC also generates a message to notify the mobile station (MS) via an FPC_SUBCHAN_GAIN parameter that the MS is to expect a change in the power of the punctured bits by the same amount indicated to the BTS(s).
At the BTS(s), each BTS monitors for receipt of an FPC_SUBCHAN_GAIN notification from the BSC. Upon receipt of such an FPC_SUBCHAN_GAIN notification, the BTS changes the power on all outgoing punctured power control bits to reflect the change of the FPC_SUBCHAN_GAIN. There may or may not be an action time associated with this command. In other words, the BSC may notify the BTS(s) that they should update the gain on the power control subchannel at some time relative to system time. This is to give the mobile station time to receive and process the message informing it about the change in the power control subchannel gain. Generally, if the mobile and system are synchronized in such a fashion, the performance is optimized. If the BTS(s) were to increase/decrease the gain right away, the mobile station will ask the BTS(s) to power down/up, as it will classify a change of subchannel gain as a change in the channel characteristic rather than a change in the system settings.
At the MS, the MS monitors for an arrival of a notification message requesting the MS of a change to the FPC_SUBCHAN_GAIN. Upon detection of such a notification of a change to the FPC_SUBCHAN_GAIN, the MS determines from the FPC_SUBCHAN_GAIN parameter what the required change of power to the punctured bits will be.
According to the IS 2000 standard, the FPC_SUBCHAN_GAIN parameter is a five bit number, a least significant bit indicative of a 0.25 dB increment of change. At present, the standard allows only positive numbers, i.e., the five-bit number is not in two""s complement. As a result, the gain on the power control subchannel is either equal to the power of the rest of the forward traffic channel bits at full rate or greater.
Subsequent to determining from the FPC_SUBCHAN_GAIN parameter the power of the punctured bits, the MS takes appropriate action to either adjust a current target Eb/No (Eb/No being representative of a bit-to-noise spectral density ratio or normalized energy per bit) to reflect the change, or scale the measured Eb/No for every power control group additively to account for the change.
The mobile station uses the target Eb/No as a means of providing fast forward link power control feedback to the network. If the measured Eb/No changes relative to the expected target Eb/No, the mobile station can provide the appropriate feedback. Consequently, changing the gain on the subchannel (which is what the MS uses to measure the Eb/No) without informing the MS will disrupt the power control feedback process.
In connection with wireless communications, diversity gain is a power level differential characteristic of wireless communications encountered during the adding or dropping of a link between one or more base stations (BTSs) and a mobile station (MS). As a result, this disrupts the power control feedback process. For instance, when the system enters into two-way soft handoff from one way soft handoff, the power required per sector (or cell) is not simply halved. This would be the case if there were no diversity gain. With diversity gain, however, the total power required from both sectors is less (in some cases much less) than the power required from one sector alone if the mobile station was not in soft handoff with the other sector.
Prior methods for solving the problem of diversity gain in a CDMA network have included a base station ignoring power control commands up/down, for a very short period when decreasing/increasing a corresponding base station controller (BSC) active set. The BSC active set is a set of identities that represent a number of sectors (e.g. up to six sectors) that the mobile is currently using to receive and transmit information to/from the network. For example, if the active set contains the identity of four sectors, then the corresponding four sectors are currently demodulating information transmitted by the mobile (providing selection diversity) and also transmitting to the mobile. The mobile may then choose to demodulate energy from all sectors or some sectors within the given cell or neighboring cell(s), depending on the number of RAKE fingers at the mobile station and the relative strengths of all the distinct signals coming to the mobile. By ignoring the power control commands, the base station allows the SNR threshold to settle and catch up to the diversity gain. However, such a method consumes valuable time, is inefficient and has the potential to reduce a stability of the CDMA system.